The present invention relates to a magneto-optic recording structure comprising both magnetic and magneto-optic layers and more particularly to such a structure in which both the magnetic and magneto-optic heads record from the same side of the structure, allowing a double-sided disk structure.
U.S. Pat. No. 4,694,358, assigned to the assignee of the present invention, discloses a recording medium comprising both a magnetic recording layer, to be written on conventionally by means of a conventional magnetic recording head, and a magneto-optic layer, to which the magnetically written information can be thermomagnetically transferred by the passage of a focussed laser beam. This magneto-optic recording structure overcomes the writeover problem of magneto-optic recording by avoiding the typical delay of one revolution of the disk for an erase pass prior to a write pass. The structure therefore achieves the high track density associated with optical recording while eliminating the writeover problem.
In U.S. Pat. No. 4,694,358, the contents of which are hereby incorporated by reference, a magneto-optic layer (or layer structure) is sandwiched between a transparent substrate and a magnetic recording layer. A magnetic head adjacent the magnetic layer is used to record data in the magnetic layer and a focussed laser beam disposed adjacent the transparent substrate is used to read and record the magneto-optic layer. Thus, the MO layer is addressed by the laser beam through the substrate to cause thermomagnetic transfer of information from the magnetic layer to the magneto-optic layer or structure, or to cause (at reduced laser power) the readout of information by means of the polar Kerr effect.
Although such a structure eliminates the writeover problem, it does not allow the use of improved substrates such as aluminum which are opaque or the use of double-sided disks. In order to use an opaque substrate or make a double-sided disk, the laser beam and the magnetic head would both have to be on the same side of the substrate. However, because a laser beam cannot penetrate an opaque substrate or a conventional magnetic layer, the magnetic layer would have to be sandwiched between the substrate and the magneto-optic layer structure.
Conventional wisdom teaches against such an arrangement. The flying height and the distance to the magnetic recording layer must be very small, i.e., a fraction of a micrometer or a few microinches, particularly at high density. Therefore, intervening layers of materials of any kind are detrimental to resolution. According to an article by Mallinson et al (1984) IEEE Transactions on Magnetics, MAG-20(3): 461-467, the contents of which are hereby incorporated by reference, there is a well-known reproducible spacing loss law of 55 decibels (a factor of five hundred) per wavelength of spacing between the head and the magnetic recording medium. This spacing loss applies to both vertical and longitudinal magnetic recording media. Although the spacing loss discussed by Mallinson refers to "reproduction" or magnetic playback of magnetically recorded signals, it is clear from the fundamental physics of magnetic recording that similar spacing losses must apply to magnetic recording as well as playback. In a recording mode, the degradation would result in increased noise or peak shift, rather than reduced signal.